Printed wiring boards have long been used to interconnect electronic components. As electronic components have increased in complexity, the interconnections have been made by multilayer printed boards.
Multilayer printed boards for high frequency or narrow pulse width signals require transmission line construction for controlled impedance to preserve the signal. Transmission lines in microstrip configuration can be used when all the required signal conductors can be accommodated on one or two signal conductor layers. A typical microstrip configuration has two internal conductor layers, a ground plane and a voltage plane (usually continuous sheets of metal with suitable clearances), and two outer signal conductor layers which are separated from either the ground or the voltage plane by dielectric material of controlled thickness and dielectric constant. A stripline configuration is used when the printed wiring board design requires more high frequency or narrow pulse width signal conductors than can be accommodated in two conductor layers. In the stripline configuration, each signal plane or signal conductor layer is placed equidistant to two parallel ground planes.
VLSI (Very Large Scale Integrated circuits) in surface mounted component packages permits closer placement of the components on a printed circuit board than can be achieved with plated-through hole component mounting. Close placing of the components permits shorter conductors and shorter component leads than plated-through hole component mounting. Short conductors and component leads are essential to maintain the integrity of high frequency and narrow pulse width signals.
In many multilayer printed boards, signal conductors are placed on a grid. A single, signal conductor layer has wireways or potential conductive paths which are parallel and equidistant from one another. A parallel signal conductor layer has wireways in a direction orthogonal to the first layer. Connections between the conductor layers are made using plated-through holes. In order to route a signal conductor from one land or terminal point on one wireway to another land or terminal point on a second wireway, the conductor must be routed through plated-through holes to a layer having orthogonal wireways to change direction or cross parallel conductors. The plated-through holes block the wireways available for other signal conductors. Blocked wireways means less available wire-ways for subsequent conductor routing, more frequent direction changes and longer signal conductor paths. Also, each plated-through hole has a capacitance of approximately 0.5 to 1.2 picofarads, so a series of plated-through holes changes the impedance of the conductor path.
Wire scribing as a method of preparing discrete boards was first taught by Burr in U.S. Pat. Nos. 3,674,914 and 4,097,684. In these patents, Burr taught the use of insulated wires so that wire scribed signal conductors can cross one another in the same layer without changing to another conductor layer. That reduced the number of plated-through holes required, increased the available wireways and shortened the signal conductors compared to multilayer printed boards. U.S. Pat. No. 4,097,684 taught termination of the wire scribed conductors in plated-through holes. Using Burr's wire scribing method the impedance of one insulated wire crossing another was found to be less than 0.03 picofarads, much less than the impedance of plated-through hole, reducing the impedance compared to multilayer printed boards.
Wire scribed interconnection boards made according to Burr use plated-through holes only for component mounting and termination of discrete wire conductors. Such boards are ideally suited for the high density conductor routing for dual-in-line packages (DIPs) and other plated-through hole mounted components.
Lassen in U.S. Pat. Nos. 4,500,389, 4,541,882 and 4,544,442 taught methods of manufacturing high density, wire scribed interconnection boards particularly suited for surface mounted components. The surface conductor layer for mounting components consisted of (a) conductive pattern or lands for connection and attachment to the surface mount components and short conductors connecting the lands to laser drilled plated-through holes. The plated-through holes in turn connect to an internal, wire scribed, signal conductor layer.
The interconnection boards according to Lassen have plated-through holes from the surface conductor layer to the signal layer. To reduce the number of wireways blocked by the plated-through holes, Lassen teaches the use of small laser drilled holes which take up less room than conventional plated-through holes. Up to this time the interconnection boards produced by the Lassen procedures have the highest conductor density available in commercial production, about 50 cm of conductors per square cm of conductor layer (125 in/in.sup.2).